kVA to Amps: The Conversion Formula, Worked Charts and CT Selection

To convert kVA to amps, divide the apparent power in kilovolt-amperes by the voltage, and by the square root of three as well on a three-phase supply. Single phase: amps = kVA × 1000 ÷ volts. Three phase: amps = kVA × 1000 ÷ (1.732 × volts). At 400 V three-phase, one kVA is 1.44 A, so 100 kVA is 144 A. At 230 V single phase the same 100 kVA is 435 A — three times the current for the same apparent power, which is why the two formulas must never be mixed.

Where the square root of three comes from

On a three-phase supply the three line currents are separated by 120 electrical degrees, and their vector sum is 1.732 times the current in any one line. The formula is not a rule of thumb to be memorised; it is the geometric consequence of sharing the same apparent power across three conductors.

That is also why a three-phase circuit needs only one third of the copper of a single-phase circuit for the same kVA. The current per conductor falls by 1.732 and the number of conductors rises from two to four, so the total conductor mass still drops.

The practical consequence for a buyer is the opposite of a saving: if you apply the single-phase formula to a three-phase nameplate, the current you calculate is 1.732 times too high, and you will oversize the meter, the current transformer and the cable. Apply it the other way round — a three-phase formula on a single-phase circuit — and the calculated current is 1.732 times too low, which is the expensive direction, because the metering then runs into overload and the CT saturates.

kVA to amps at the voltages you actually meet

The table below is calculated with the formulas above, not measured. The 230 V column is single phase; the 400 V and 415 V columns are three phase line-to-line.

Note how little the 400 V and 415 V columns differ. The 3.75% gap in voltage produces the same 3.75% gap in current, so the choice between them rarely changes the equipment selection. What does change it is the difference between single and three phase, which is a factor of 1.73 and moves the answer across whole product families.

From amps to the meter: the 80 A ceiling

Direct-connected meters are limited by the current that can pass through their terminal block. YOMIN's ym-0058, a three-phase four-wire DIN rail meter with a 3×5(80 A) rating, WiFi communication and a digital display, carries the load current through the meter itself. At 400 V three-phase that 80 A ceiling corresponds to 1.732 × 400 × 80 ÷ 1000, or about 55 kVA.

Above roughly 55 kVA the circuit moves to a current transformer, and the meter becomes a 5 A device reading a scaled signal. This is the point where a kVA-to-amps calculation stops being an academic exercise: the answer decides whether the installation needs a direct meter or a CT-operated one, and the two are not interchangeable after the panel is built.

The full wiring difference between those two arrangements is set out in the energy meter wiring diagram guide.

Choosing the CT ratio from the current

Once the current is known, the transformer ratio is the next decision. The primary rating must sit above the maximum demand, and the secondary is almost always 5 A in this class of equipment. The table gives the three-phase load that each standard primary corresponds to at 400 V.

Two hardware details sit behind those model numbers. The ym-0073 is a split-core transformer: the core opens, so it can be clamped around a cable that is already terminated, which is what makes a retrofit possible without dismantling the feeder. The ym-0064 is a 720 V AC, Class 1.0 metering transformer rated for 3 kV AC for one minute between windings and earth — the insulation figure that matters when the primary sits on a 690 V system.

Ratio selection itself is covered in more depth in the guide to choosing a current transformer, including what happens when the primary is chosen too close to the running load.

kVA is not kW, and the difference changes the current

Everything above converts apparent power. A load's real power in kW is the kVA multiplied by the power factor, so a 100 kVA transformer feeding a plant at 0.8 power factor delivers 80 kW. The current, however, is set by the kVA, not the kW — the cable and the meter carry the apparent current whether or not the load converts all of it into useful work.

This catches out buyers who start from a motor list in kW. A 90 kW motor at 0.85 power factor and 0.9 efficiency draws about 106 kVA, not 90, and the feeder must be sized on the larger figure. The kVA versus kW comparison works through that correction.

What to put on the enquiry

Send those five lines and the specification is unambiguous. Send only “2000 amp meter” and the supplier has to guess whether you mean a 2000 A direct device, a 2000/5 CT, or a 5 A meter to be used with one.

Frequently asked questions

How many amps is 100 kVA?

It depends on the voltage and the phase count. At 400 V three-phase, 100 kVA is 144.3 A. At 415 V three-phase it is 139.1 A. At 230 V single phase it is 434.8 A.

Should I use 400 V or 415 V in the formula?

Use the nominal voltage your supply actually declares. The difference between the two is 3.75% in both voltage and current, which rarely changes a product selection but does change the displayed current on a report.

Why does my calculated current not match the meter reading?

Three usual causes: the load is not balanced across the phases, so one line carries more than the average; the meter is CT-operated and the reading is the secondary current; or the calculation used kW where the nameplate gave kVA.

Can I use the single-phase formula on a three-phase supply?

No. It omits the square root of three, so it overstates the current by 73%, which leads to an oversized meter, an oversized CT and an oversized cable.